The purpose of this study is the preparation of cross-linked hemoglobins of two kinds: (a) Intermolecular cross-linking giving rise to high molecular weight "polyhemoglobins", and (b) Intramolecular cross-linked hemoglobin tetramers of molecular weight 68,000. For (a) the reagent used is deithyl malonimidate and for (b) the reagent is dimethyl sebacimidate. These products will be tested as potential plasma protein extenders since they are less capable of dissociation than unmodified hemoglobin. In addition, the relative positions on each polypeptide chain of surface lysyl groups, which react with the reagent, will be determined in the intramolecular cross-linked tetramer.